1. Technical Field
The present invention relates to an image reading device and a method for reading an image.
2. Related Art
There is a CIS (contact image sensor) scanner having an image sensor formed by a plurality of sensor chips arranged in a line in a main scanning direction. The image sensor of the scanner produces analog data including a reference portion (which may be called a “field through portion”) and a data portion, and an A/D converter of the scanner produces digital data indicating a pixel value by using a difference between the reference portion and the data portion.
As opto-electric conversion elements of the sensor chips are not uniform, the reference portions of a waveform corresponding to pixels include offset differences among one another. The scanner reduces the offset differences among the pixels by performing clamping (fixing the reference portion of the waveform at a constant voltage by adding a dc component to the signal waveform).
JP A-H05-3546 discloses an example of the related art.
Among the different sensor chips, however, the offset differences may be great. If the offset differences are great, fixation of the voltage by clamping may not be able to keep up, and thus cannot reduce the offset differences enough. In such a case, quality of image data output may be degraded.